Method of making electret

ABSTRACT

A method of making an electret by annealing a corona charged dielectric film on a metal plate is disclosed.

United States Patent [191 Kodera 5] Dec. 9, 1975 METHOD OF MAKINGELECTRET [75] Inventor: Yoichi Kodera, Yokohama, Japan v ReferencesCited [73] Assignee: Sony Corporation, Tokyo, Japan UNITED STATESPATENTS 3,660,736 5/1972 lgarashi et a1. 307/88 ET X [22] July 19743,794,986 2/1974 Murayama 307/88 ET x [21] Appl. No.: 485,112

Primary Examiner-C. W. Lanham Assistant Examiner-Joseph A. Walkowski[30] Foreign Apphcatlon Prlonty Data Attorney, Agent, or Firm-Lewis H.Eslinger; Alvin July 5, 1973 Japan 48-76027 Si d -b d [52] US. Cl29/592; 179/111 E; 307/88 ET;

317/262 A [57] ABSTRACT [51] Int. Cl. H01S 4/00 A method of making anelectret by annealing a corona [58] Field of Search 29/592, 592 E;317/262 A; charged dielectric film on a metal plate is disclosed.

307/88 ET; 179/111 E, 100.41 B; 310/82, 8.8, 9.1, 9.2, 9.7; 340/173 PP;427/39 8 Claims, 1 1 Drawing Figures L3 9 4 Vol 110136.

Source,

U.S. Patent Dec. 9 1975 Sheet 3 of3 3,924,324

METHOD OF MAKING ELECTRET BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates generally to a method of making anelectret, and more particularly is directed to a method of making anelectret with a single electric charge or a monocharge electret.

2. Description of the Prior Art Recently, a thin film electret is usedas an oscillating plate or vibrating diaphragm of a microphone, speakeror the like. As the material for the thin film electret, by way ofexample, a high polymer film such as Teflon FEP (Trade name) consistingof a copolymer of tetrafluoro ethylene-hexafluoro propyrene or TeflonTEF (Trade name) is used.

Various methods have been known in the art to produce an electric chargeon a dielectric material. In general, a dielectric material is grippedbetween two electrodes and subjected to heating process to make it as anelectret by a heterocharge, which is known as a socalled thermo-electretmethod. In this case, when an electret, which is charged with positiveand negative charges on its both surfaces, is used as an electrode layeris attached to one surface of the electret and the electric chargeproduced on the other surface thereof is utilized.

If an electret produced by the above prior art method is employed in anear-speaker, two of such electrets are used in such a manner that theyare mounted on fixed electrodes or back electrodes with apertures toface the charges of the electrets of the same polarity or sign, and anelectric conductive vibrating diaphragm is located between the twoelectrets at their central portions from a spacial point of view tovibrate'the vibrating diaphragm with an input signal applied to an inputtermi-,

nal.

However, such a method can not be free from the following defect. Thatis to say, when an dielectric film adhered to the back electrode issubjected to an electret making process, it is difficult to make anelectret with a homogeneous electric charge due to the distortion of theback electrode caused by heating carried out in the electret-treatment,a time lapse change of the adhesive between the dielectric film and theback electrode, and a plurality of apertures bored through thedielectric film formed by a corona discharge. Further, when a film whichis already made as an electret is attached to the back electrode, it istrouble to aligne the apertures of the film with those of the backelectrode and there are a charge decay and so on due to a stain of thefilm.

Another method is also known in the art in which an electron beam withhigh energy is radiated to an dielectric film from a Van de Graaffgenerator. With such a method, upon an initial irradiation of theelectron beam, a positive electric charge is produced on the surface ofthe dielectric film facing the electron beam irradiation, while anegative electric charge is produced on the other surface thereof. Aftera predetermined time period has lapsed, whole the film is charged with anegative electric charge entirely. This method, however, requires a muchgreat size of an apparatus which is not preferred from a practical pointof view, and the dielectric film is apt to be broken due to theirradiation of the electron beam, which causes the deterioration of aproduced electret in characteristics.

SUMMARY OF THE INVENTION According to this invention, there is proposeda method of making an electret, in which a high polymer film with noelectrode is placed on a metal table and is charged with a negativecorona, and then the high polymer film is subjected to an annealingtreatment to be an electret with an electric charge of a single sign ora monocharge electret.

Further, after a corona discharge the film is peeled off from the metaltable once, and the film is mounted on the table ,with its surface,which is charged with a positive charge, being faced to the surface ofthe table, and then the film is subjected to the annealing treatment tocarry out a charging process with high efficiency for a short timeperiod.

With this invention, it is possible that a high polymer film is chargedat its both surfaces with a negative charge by a simple construction ofa device with the utilization of acorona discharge.

In a practically used charging device, a negative corona discharge isapplied to a non-polar high polymer film disposed on a metal table, andthereafter the film is annealed to obtain a mono-charge electret film.

In this invention, a stylus electrode is disposed apart from the surfaceof the film by 15mm and supplied with a voltage of 4 to 8KV (kilovolts).By the exposure of about 30 sec (seconds), an electret with an initialsurface charge density of 10 7 to 10'8 C/cm (coulombs per squarecentimeter) is obtained.

Thereafter, the electret is, at it is, annealed at a temperature ofabout C to have a negative surface charge density of l09 C/cm and thenpeeled off from the metal table to be a monocharge electret film with anet charge as maintained.

Further, if the film peeled off from the metal table is again mounted onthe metal table with its positive charged surface in contact therewithand is then subjected to an annealing treatment, a monocharge electretfilm can be obtained for a short time period.

If after the-corona discharge an annealing treatment is applied in theabove method, the fact that a charged surface by the'corona discharge istransferred from the discharging side to that of the metal table isshown by measuring a thermoelectric current spectrum. As a result, anelectrified or charged film is obtained which is homogeneous and stableas compared with a film naturally charged by a static electricity.

Other objects, features and advantages of this invention will becomeobvious from the following description taken in conjunction with theaccompanying drawmg.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1A to IE, inclusive, are diagramsshowing a process of one of the methods according to this invention;

FIGS. 2A to 2C, inclusive, are diagrams showing a process of another ofthe methods of this invention;

FIGS. 3 and 4 are respectively schematic theoretical diagrams showinghead-phones used for the explanation of the invention; and

FIG. 5 is a graph showing the surface charge densitytime lapsecharacteristics used for the explanation of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the prior art, when anelectret is used in a headphone, as shown in FIG. 3, a pair of opposedelectrets 1 and 2 are attached to a pair of 'fixed electrodes or backelectrodes 4 and 5 which have bored there through apertures 3,respectively, and an electrically conductive vibrating diaphragm 6 isdisposed between the electrets 1 and 2 to vibrate in responseto an inputsignal applied to an input terminal t. With such a' prior artconstruction, the two electrets 1 and 2 are used in a single transducer,and upon the manufacture thereof two dielectric films to be made as theelectrets 1 and 2 are attached to the back electrodes 4 and 5,respectively, which is thereafter subjected to an electret treatment.However, there may be a difficulty in such an electret treatment due tothe type of material used in the back electrodes. That is, there areproblems of the distortion of the back electrodes and the deteriorationof the adhesive used to attach the dielectric films to the backelectrodes which may be caused by the heating during the electrettreatment. Further, in the case of an electret treatment wherein thedielectric film is charged on its surface by a corona discharge, ifthere are many apertures in the dielectric film, there is theunfortunate possibility that a homogeneous charge production is notobtained on the surface of the dielectric film.

In order to avoid such a defect, there has been proposed a method withwhich a dielectric film is charged firstly to be an electret andtheelectret is attached to the back electrode. However, this methodrequires complicated procedures in that a plurality of apertures arebored through the electret film and the electret film is adhesivelyattached to the back electrode with the apertures of the electret filmand those of the back electrodes being aligned with each other. In thisprocess, there is a fear that the electret film is blured and thesurface charge thereof is decreased.

, In order to eliminate such a defect, it may be considered that avibrating diaphragm is made as an electret. However, in such a case thevibrating diaphragm must have a monocharge that is, an electric chargeof a single polarity so as to be operative as a vibrating diaphragm. Byway of example, if such a vibrating diaphragm is used in a head-phone,as shown in FIG. 4, the vibrating diaphragm 7, which is made to be anelectret to have, for example, only a negative charge, is centrallydisposed and the back electrodes 4 and 5, each of which has formedtherethrough a plurality of apertures 3, are disposed at the both sidesof the diaphragm 7 in opposed relation with each other. Thus, theheadphone becomes simple in construction.

Hereinbelow, the method of making an electret according to thisinvention, which has a negative charge on its both surfaces and isstable, will be described in conjunction with the drawing.

As shown in FIG. 1A, a dielectric film or a high polymer film 10 to bemade as an electret is mounted on a metal table or plate 1 1 and astylus electrode 12 is positioned above the high polymer film 10 apartfrom its upper surface 10A by a predetermined distance d. Apredetermined high voltage is applied across the stylus electrode 12 andthe metal plate 11 from a high voltage source 13 with the metal plate 11being connected to a positive electrode while the stylus electrode 12being connected to a negative electrode to carry out a corona dischargebetween the stylus electrode 12 and the high polymer film 10 and tocharge the high polymer film 10 with negative charge on its uppersurface 10A.

Thereafter, as shown in FIG. 1B,the high polymer film 10 is peeled offfrom the metal plate 11. At this time, a discharge occurs between apeeled off surface 10B of the polymer film 10 and an upper surface 11Aof the metal plate 11, and hence the surface 10B of the high polymerfilm 10 is charged with a positive charge. Thus, as shown in FIG. 1C,the high polymer film 10 is charged with negative and positive chargeson its surfaces 10A and 10B, respectively.

Thereafter, the high polymer film 10 is again placed on another metalplate 14, with the surface 10B having the positive charge being incontact with the plate, and is heated in a furnace 15 for a constanttime period and then cooled. In this case, it may be preferred that themetal plate 14 is grounded, as shown in FIG-1D. In the figure, referencenumeral 16 indicates a heater. In this process, the heating temperatureis selected higher than the glass transition temperature of the highpolymer film 10, for example, more than C for the case that the highpolymer film 10 is made of Teflon FEP. If the heating temperature is toohigh, there may be a fear that the film is distorted or molten.Accordingly, it is necessary that the heating temperature is selectedlower than the melting point of the film.

Thus, an electret film 17 which is charged with the negative charge onits both surfaces 10A and 10B is produced as shown in FIG. 1E.

That is, it is ascertained by measuring the electric charge on thesurfaces 10A and 10B of the high polymer film 10 that the surface 10B,which is charged with the positive charge before heating, is chargedwith the negative charge by suitably selecting the time interval of theheating, while the amount of the negative electric charge stored in thesurface 10A before the heating is decreased somewhat, but the surface10A has still the negative charge, that is, both the surfaces 10A and10B carry the negative charge. The amount of initial charge by thecorona discharge is much greater than can be at- .tained by the thermoelectret method, and the charge where 0 represents the charge densityand s the dielectric constant in vacuum. Since the critical discharge ofa uniform electric field in atmosphere is 30KV/cm at normal temperatureand pressure, the maximum charge density of such a charged body becomesto 5.3 X 10" C/cm, which is smaller than that where the high polymeffilm is charged with positive and negative charges on its both surfacesor where the film is provided'with a back electrode. In other words, asmall charge density can provide the necessary electric field.

FIGS 2A to 2C'show another'niethod of the invention. With the secondmethod,-the high "polymer film to be made as an electretis mounted onthe metal plate 11 and the stylus electrode 1.2 is locate'dabove. thefilm 10 apart from its upper surface 10A by a predetermined distance inopposed relation-Apredetermined high voltage is applied across the.stylus electrode 12 and the metal plate 11 from the high voltage source13 with the metal plate 11 being positivenand the stylus I electrode 12being'negativezto produce a 'negativeelec; tric charge on the uppersurface 10A of thezfilm 10 by the corona discharge (refer to,F-IG..2A-).--

The film .10 andthe metal plate-=1 1"? are. located in the heatingfurnace 15' without the film- 10 first being peeled off from the metalplate ;l .l as .before; and the film 1,0 is heated (annealed) gtherein.for a constant time interval. Thereafter the film- 10, is cooledln, thiscase, the heating treatment is carried out longer than. that in the caseof the first method (FIGS. lA lD).

As a result of measuring the-charge .on both the surfaces 10Aand:l0B.;-of ,th,e;;high polymer film 10 after cooling, it isascertained that, by suitably. s ele cting.the heatingtime interval, anelectret 18 (FIGQC), which is charged with ,the negative charge ,on itsboth surfaces 10A and 10B or carries a single sign of anelectric charge(as desired), can be obtained. V

Examples of the present nt'ion will belprovidjed.

EXAMP A Teflon FEP ioof ao After the corona discharge is finished;thefilm' is peeled off from the metal plate 1 l'a nd' placed ontheelectrode of an electric charge measuringapparatusTAfter measuring thecharge amount of the film, it is8 X 10 C/cni at its portion opposing thestylus electrode 12. After turning over the film'ahdnieas'urin'g thecharge ambunt on the surfaceopposite to the discharged surface, itis+7;8 X' 1 0 Clem. 'Aftertlie measurement, 45

the film is mounted on the metal plate with its positive chargeds'urfaceffacin'g the plategand heated at 155C for'a'timeintefval ofte'r'i minutes/When the charge amount of the film after heating ismeasured, it is l .9

X 10 C/cm on the surface of the film which was charged with the negativecharge initially and 2.3 X 10" C/cm on the surface of the film which wascharged with the positive charge initially, that is, both the surfacesof the film are charged negatively.

EXAMPLE II of 30 seconds. After the corona discharge is finishedf thefilm is peeled off from the metal plate 11. Then, the film is placed onthe second metal plate 14 with its corona-discharged surface facingupward and heated at traflu'oroe- 160C fora-time interval of 20 minutes.It is ascertained by the aforedescribed measurement that the chargedamount after heating is 4.7 X 10 C/cm on the corona-discharged:surfaceand 4.0 X 10' C/cm 5 on the surface that was-in-contact with the metalplate, that-is, both the surfaces are charged negatively. The change inthe amount of the charge of the film with respect to the time lapse isshown in FIG. 5 in which the ordinate represents the surface chargedensity Q in multiple of 10 C/cn and the abscissa the elapsed time T inday, when the film is left at a room temperature. the'graph' of FIGsS, acurve I shows the charge on the corona-discharged surface and a curve 11shows the charge-batheopposite-surface. Asmay be obvious from thecharacteristic of the graph, it is noted that the charges on-b'oth thesurfaces of the film are negative and' settled after 10 days and..thesettled negative charge is""stable without" being changed with furthertime elap'se'ia 1...; 23 I EXAMPLE ill I A Teflon FEP film with an areaof 1.5 X 1.5 cm and athickness of 50 um: is mounted on the metal plate11 and the stylus electrode 12 is located above the film 2:5 "aparttherefromby 15mm. A voltage of 6KV is applied' to the tip end of thestylus electrode 12 to carry OUI-IheFCQI'OHQ discharge for a timeperiod-of seconds..After. the corona-discharge is finished, the film isheated at 150C for a time interval of 1.5 hours without :the filmbeing-peelef offfrom the metal plate 11. It is ascertained that thecharge amount of the film after i-lieatingi's --11 .3. X 10" Clcrn onits corona-discharged surface and .2.4 X 10 Clcm on the surface whichwas in contact with the metal plate, both surfaces thus*negatively'charged. l v I ANorngti xAMPLmay p Teflon-.FEPjilm'with anarea of 1.5 X 1.5 cm and rathicknessbfSOpm is placed on the metal plate1 1 and the stylus electrode 12 is located above the film aparttherefrombyxdSmmJiA voltage of 6KV is applied to the tip end of thestylus electrode 12 for atime interval of30 secondsto carry out thecorona discharge. As a result ofthis, an electric charge of 5.5 X 1075C/cm --is produced Orr-the. corona-discharged surface of'the film.T-hen,-;the.-:=film ispeeledzoff from the metal plate 11 and againplaced on the metal plate. At this time, "the; peeled'ioff surfaceof-the film is charged with the positiv'e charge; Thereafter, a metalmeshis disposed betweert'the.tipyend-of-the;s.tylus electrode and themetal plate and a voltage of about 100V is applied to the metal meshrelative to the metal plate, while a voltage of 6KV is applied to thetip end of the stylus electrode to carry out the corona discharge. Inthis case,

" the amount of the charge is 5.5 X 10" to 7 X 10 its reproducibility isdeteriorated.

FURTHER EXAMPLE (b) A separate specimen (Teflon FEP film), which ischarged by the similar corona discharge to'that of the 765 Example I,has the charge of 8.6 X 10 C/cm on its coronadischarged surface and thatof +8.3 X 10" C/cm on its surface in contact with the metal plate Next,the film is placed on the metal plate 14 with it:

negatively charged surface in contact therewith and heated under thesimilar condition of the Example I or at 155C for 10 minutes. Whenmeasuring the charge on both the surfaces of the film, the surface whichis charged with the negative charge initially is charged at 5.7 X 10'C/cm and the surface which is charged with the positive charge initiallyis charged at +2.1 X 10 C/cm, that is, the sign of the charges is notchanged.

THE OTHER EXAMPLE (c) A separate specimen (Teflon FEP film), which ischarged by the similar corona discharge to that of the Example I, ispeeled off from the metal plate. Then, the film which is chargedwith-the negative and positive charges on its both surfaces,respectively, is heated without being in contact with the metal plate.After heating, the measurement of the charges shows that the positivelycharged surface remained positive and the other surface remainednegative. That is, the film with a single sign of charge can not beobtained.

If a TeflontFEP film with a thickness of 50pm is placed ona metal plateand a negative charge is produced on the surface of the film of 5.5 X 10C/cm, this charge is below its critical stability. However, since thestable charge is on the order of X C/cm when the film is peeled off fromthe metal, a discharge occurs on the peeled off surface upon the peelingoff to cause the production of the positive charge. Accordingly, if thefilm on the metal plate is charged at lower than about 5 X l0' C/cm"initially, theoretically there-appears no discharge on the peeled offsurface when the film is peeled off from the metal plate. Thus,

there must be obtained an electret with a monocharge or a single sign ofelectric charge. However, from a practical point of view, itsreproducibility is low, and the reproducibility and the amount of acharge in the case of heating treatment are less than desired ascompared with the former case. Further, in'order to shorten the timeinterval of heating treatment, it is better that after the coronadischarge the film is peeled off from the metal plate once.

In the above example, a Teflon FEP film is used as a high polymer film,but even if a Teflon TFE film,a Tefzel film and the like are used as thehigh polymer film, an electret film with a single sign of an electriccharge can be also obtained.

As described above, with this invention, by taking the step of chargingthe surface of a high polymer film with the corona discharge and thestep of heating the high polymer film after the corona discharge, anelectret having a monocharge, that is, a single sign of an electriccharge such as a negative electric charge, can be easily obtained.Further, an apparatus for performing the method of this inventionrequires no complete redesign and thus can be the apparatus used formaking the prior art electret.

It may be obvious that many variations and changes on said electrode,heating said charged dielectric film.

2. A method of making a mono-charge electret as claimed in claim 1 inwhich said stepof heating is carried out at a temperature higher thanthe glass transition temperature of said dielectricfilm.

3. A method of making a mono-charge electret as claimed in claim 1 inwhich said dielectric film is made of fluoride. 1 I

4. A method of making a mono-charge electret as claimed in claim 1 inwhich'dielectret is charged by a negative corona discharge.

5. A method of making a mono-charge electret, comprising the steps ofcharging a dielectric film which is mounted on a metal electrode bysubjecting said film to a corona discharge to thereby charge one surfaceof said film with the polarity of said corona discharge; peeling offsaid charged dielectric film from said electrode; placing saiddielectric film on a metal backing with the other surface of said filmin contact with said metal backing; and, vwhile said film is on saidmetal backing, heating said dielectric film. I

'6. A method of making a mono-chargeelectret as claimed in claim 5, inwhich said step of heating is carvried out at a temperature higher thanthe glass transition te'mperatureof said dielectric film. I

7. A method of making a mono-charge electret as claimed in claim 5 inwhich said dielectric film is made of fluoride. 7

8. method of making a mono-charge electret as claimed in claim 5 inwhich said dielectret is charged by a negative corona discharge.

' is it l l ll

1. A method of making a mono-charge electret, comprising the steps ofcharging a dielectric film which is mounted on a metal electrode bysubjecting said film to a corona discharge; and then, after said coronadischarge is completed and while said film is still mounted on saidelectrode, heating said charged dielectric film.
 2. A method of making amono-charge electret as claimed in claim 1 in which said step of heatingis carried out at a temperature higher than the glass transitiontemperature of said dielectric film.
 3. A method of making a mono-chargeelectret as claimed in claim 1 in which said dielectric film is made offluoride.
 4. A method of making a mono-charge electret as claimed inclaim 1 in which dielectret is charged by a negative corona discharge.5. A method of making a mono-charge electret, comprising the steps ofcharging a dielectric film which is mounted on a metal electrode bysubjecting said film to a corona discharge to thereby charge one surfaceof said film with the polarity of said corona discharge; peeling offsaid charged dielectric film from said electrode; placing saiddielectric film on a metal backing with the other surface of said filmin contact with said metal backing; and, while said film is on saidmetal backing, heating said dielectric film.
 6. A method of making amono-charge electret as claimed in claim 5, in which said step ofheating is carried out at a temperature higher than the glass transitiontemperature of said dielectric film.
 7. A method of making a mono-chargeelectret as claimed in claim 5 in which said dielectric film is made offluoride.
 8. A method of making a mono-charge electret as claimed inclaim 5 in which said dielectret is charged by a negative coronadischarge.